1. Field of the Invention
This invention relates to improvements in ring laser gyroscopes, and more particularly, to improvements in output optics and detectors for use in ring laser gyroscopes.
2. Description of the Prior Art
As is known in the art, a ring laser gyroscope is a nonconventional gyroscope in which laser light is circulated in counter rotating directions in a ring about an axis of rotational sensitivity. Depending upon the rate of rotation of the ring about the axis, the counter rotating light beams effectively change in frequency. Ordinarily, an output is derived from the gyroscope at a reflective corner of the ring to determine the intensities of the counter rotating beams. An example of this output derivation is shown in U.S. Pat. No. 3,411,849. Additionally, ordinarily at another reflective corner, another output is derived in which the beams are superimposed to develop interference fringes indicative of the frequency difference of the counter rotating beams. Thus, ordinarily separate detectors (not shown) are employed to detect the intensities of the respective counter rotating beams.
Numerous types of output optics and detectors have been suggested for use in developing outputs from ring laser gyroscopes. One optics-detector arrangement of the prior art is taught by D. M. Thymian and T. J. Podgorski, ASTIA Document AD 527867, GG1300AD01, Laser Gyro Final Report, May, 1973, pp. 7-8. Another reference showing laser gyroscope output techniques is Laser Applications, "The Laser Gyro", Vol. 1, 1971, Academic Press, pp. 138-141. Another gyroscope detector which has been advanced is described in U.S. Pat. No. 4,152,072, and is denoted generally by the reference numeral 10, as shown in FIG. 1.
The detector structure 10 includes a substrate 11 on which is carried a detector layer 12. The substrate 11 has an anti-reflection coating 15 on the front face and a multilayer dielectric reflection coating 16 on its back face. Typically, the multilayer dielectric reflection coating 16 serves as a partially reflecting mirror, allowing a portion of the light incident thereupon to pass through to the detector layer 12, and reflecting the remaining portion of the incident light back into the ring. To enable the detector 10 to detect the fringes developed, (interference fringes between the two counter-rotating beams being developed on the detector 12 as is known in the art) a mask layer 18 is provided between the detector element 12 and the multilayer dielectric coating 16. Finally, anti-reflection isolators 19 and 20 are located on each side of the mask 18.
In the operation of the detector structure 10, a fringe pattern is developed on the detector element or layer 12, dependent, in part, upon the angle, .theta., between the two counter rotating beams following the light path 23. The fringe pattern is made up of a portion of the light passing through the partially reflective surface 16, as described above. It can be seen that the reflection of the counter rotating beams is from the back face (as seen from the interior of the cavity) of the substrate 11 at its interface with the multilayer dielectric reflection coating 16, in a fashion such that the patterns produced upon the detector 12 through the mask 18 are in substantial registration.
One of the disadvantages of the arrangement exemplified by the structure 10 is that the light within the ring is caused to pass through both the anti-reflection coating 15 and the substrate 11 before being reflected from the partially reflective surface 16. In ring laser gyroscopes, it is important to minimize the losses within the ring as much as possible, so the passage of the light through these portions of the detector structure is regarded as being undesirable.
In summary, ring laser gyroscopes of the type exemplified by the structure illustrated in FIG. 1 have several disadvantages. The mirror substrate is inside the ring laser gyroscope cavity and thus contributes to cavity loss. Substrate thermal capacity may also adversely affect the thermal response time for the instrument. Also, the anti-reflection coating required on the front surface of the mirror substrate adds to cavity loss, may be required to withstand a gas discharge, and introduces more cost. Additionally, ring laser gyroscope single beam intensities are not available from this type mirror using the FIG. 1 mechanization. The single beam signals contain much information which may be used to predict, and perhaps even control, gyroscope performance. See, for example, U.S. Pat. No. 4,152,071. Comparison of the two single beam signals yields data on the differential intensity shift between the two beams produced during passage through lock-in; it also provides data on the phase relationship between the amplitude modulations on the two beams.
In contradistinction to the prior art gyroscopes which, as mentioned, derive outputs from a plurality of reflective corners of the ring (except for that taught in U.S. Pat. No. 4,152,072, but which does not derive single beam intensity signals), in my copending patent application, Ser. No. 410,790, filed Aug. 23, 1982, and entitled Ring Laser Gyroscope Readout assigned to the assignee hereof, and incorporated by reference herein, a ring laser gyroscope is described in which all of the required or necessary outputs are derived from a single reflective corner. It is for use in such instances that the detector of the present invention is directed.